1. Field of the Invention
This invention relates to the vinyl polymerization of ultraviolet (UV)-curable vinyl compositions. In one aspect, the invention relates to a class of additives useful in such a polymerization and in another aspect, the invention relates to the use of these additives to promote essentially complete vinyl polymerization of the composition.
2. Description of the Prior Art
Incomplete cure of UV-curable compositions is a problem long recognized in the art. Although this problem is manifest in many UV-curable compositions, the problem of incomplete cure with multifunctional acrylate materials is exemplary. Multifunctional acrylate diluents, such as 1,6-hexanediol diacrylate, trimethylol propane triacylate or pentaerythritol triacylate, are well-known to produce significant residual unsaturation in UV compositions. A similar but less well-recognized problem can also arise with sterically encumbered monofunctional acrylates such as dicyclopentadiene acrylate (DCPDA). Residual acrylate in cured coatings has several deleterious effects among which are changing film properties over time (films may become yellow and brittle) and objectionable odors. This latter effect is particularly true in the case of DCPDA. Although numerous approaches are known for essentially completing the cure of such UV-curable compositions, many are not effective with sterically encumbered monofunctional acrylates while others have undesirable limitations and/or features. For example, the use of various photoinitiators, comonomers, and drier metals, such as cobalt naphthenate, do not reduce the level of residual DCPDA below 5 or 6 weight percent of the cured composition. Others, such as a post-bake treatment, have limited utility. Post-baked treatment is generally limited to heat insensitive objects and requires an extra, energy-consuming process step.
U.S. Pat. No. 4,077,858 teaches the use of an aliphatic saturated ether as a promoter for essentially complete polymerization of residual unsaturation in multifunctional acrylate UV-curable compositions.